Unsaturated esters of aliphatic carboxylic acids, as defined herein, are characterized by the presence of at least one and not more than two ester groups (--COOR, wherein R is an alkyl radical preferably containing up to 6 carbon atoms) and at least one but not more than two unsaturated carbon to carbon bonds (either olefinic or acetylenic or a mixture of each) in the molecule.
The unsaturated esters of aliphatic carboxylic acids can be prepared by a great number of procedures set forth in the technical and patent literature.
Several different processes are used commercially to prepare acrylate esters including the less utilized acrylic acid. More than half of the total acrylates prepared in the U.S. employ acetylene as the starting material. The older processes react acetylene with nickel carbonyl in methyl ethyl ketone in combination with carbon monoxide under pressure. While yields and selectivity are good, nickel carbonyl is a highly toxic material.
A more complex version of the above process is to pre-absorb the acetylene in tetrahydrofuran in the presence of an alcohol and carboxylate with carbon monoxide under pressure in the presence of a nickel salt catalyst including Ni(CO).sub.4.
Other known processes react ketene (H.sub.2 C=C=O) with formaldehyde to produce Beta-propiolactone, which combines with water or an alcohol in the presence of dehydrating agents to form acrylic acid or the respective ester; or selectively oxidize propylene to acrylic acid and acrolein and further oxidize the acrolein to acrylic acid.
The extensive literature on Reppe-type chemistry* teaches that the carbonylation of unsaturated aliphatic halide may proceed either through addition of carbon monoxide to the carbon-carbon unsaturated bond to give, in the presence of a suitable hydroxylated co-reactant, the corresponding halogenated acid derivative. For example, the synthesis of chlorobutanoate esters from allyl chloride: ##STR1## FNT * See "Carbon Monoxide in Organic Synthesis" by J. Falbe, published by Springer-Verlag, N.Y. (1970), Chapter II.
Alternatively, carbon monoxide insertion into the carbon-halogen bond, with halide displacement, may occur, to give, in the presence of the hydroxylated co-reactant such as an alcohol, the corresponding unsaturated acid derivative. For example, the synthesis of vinyl acetate esters from allyl chloride: EQU CH.sub.2 =CH--CH.sub.2 Cl + CO + ROH .fwdarw. CH.sub.2 =CH--CH.sub.2 --COOR + HCl (2)
In the case of the carbonylation of allylic halides, carbonylation via CO insertion into the carbon halogen bond is documented in the literature (British Pat. No. 987,274[1965]). However, carbonylation via CO insertion with the homogeneous ligand-stabilized noble metal catalysts with Group IVB metal halide co-catalysts of this invention provides an improved process for the synthesis of unsaturated, halogen-free, acid derivatives with improved yields and selectivity to the unsaturated, halogen-free, acid derivative as, for example, in the synthesis of vinylacetate esters from allyl chloride. EQU H.sub.2 C=CH--CH.sub.2 CL + CO + ROH .fwdarw. H.sub.2 C=CH--CH.sub.2 --COOR + HCl (3)
The carbonylation of vinylic chlorides normally proceeds through Co addition to the carbon-carbon unsaturated bond (cf equation 4) to give the corresponding halogenated acid derivative, for example the synthesis of .alpha. -chloropropionate ester from vinyl chloride (German Pat. No. 1,227,023[1966]). EQU H.sub.2 C = CHCl + CO + ROH .fwdarw. ROOC--CHCl--CH.sub.3 ( 4)
in this invention, noble metal catalysts, particularly ligand-stabilized noble-metal halide catalysts with Group IVB metal halide co-catalysts, are used to catalyze the insertion of CO into the carbon halogen bond of the vinylic halide to give unsaturated aliphatic, halogen-free, acid derivatives. For example, the synthesis of acrylate esters from vinyl chloride: EQU CH.sub.2 = CH--CL + CO + ROH .fwdarw. CH.sub.2 = CH--COOR + HCl (5)
It should be noted that the instant invention is also distinguishable over previous cases* in that:
1. In the previous cases, while the same classes of homogeneous catalysts are used in conjunction with carbon monoxide and unsaturated substrates, alpha-olefins are the substrates in the previous cases rather than unsaturated aliphatic halides such as vinyl chloride or allyl chloride.
2. In the previous cases only carbon monoxide addition to the unsaturates takes place, whereas in the instant application carbon monoxide insertion into the carbon-halogen bond with displacement of halogen takes place, without addition to the unsaturated bond.
3. In the previous cases only stoichiometric quantities (2 moles) of the Group VB donor ligand, typified by triphenylphosphine, to noble metal component are used in the reaction, whereas excess (1 to 10 moles) of triphenylphosphine to noble metal component may be used in some cases for the carbonylation of unsaturated aliphatic halides. FNT * U.S. Pat. No. 3,819,669 and U.S. Pat. No. 3,919,217